Imaging Suspended Sediment Concentration in Ship Channels with Marine Electrical Resistivity

Electrical resistivity is a common near-surface geophysical measurement for geotechnical site investigations. Often, there is a need to run electrical resistivity surveys underwater, such as to inspect unknown bridge foundations or groundwater surface water interactions; however, these measurements are static where the electrodes are placed on the floor of the waterbody. It is also possible to gather marine electrical resistivity measurements, in which case a cable is towed on the surface of a waterbody, and data are continuously collected. Marine measurements are advantageous because large areas can be covered in a relatively short time; however, there is limited evidence of marine resistivity used for geotechnical investigations. In this study, we present a case history of using a towed marine electrical resistivity measurement to image sediment concentration in a deep-draft ship channel traversing a shallow bay system. The data were collected in the Houston Ship Channel (HSC) through a series of surveys before and after deep-draft vessel transits to monitor the change in electrical resistivity of the water affected by the amount of resuspended sediment. There was a relative 34.7% decrease in the water column resistivity at the channel floor between a survey following a vessel and a survey without a vessel; the change in resistivity between the surveys was significant. By contrast, there was only a 2.7% change in resistivity, which was not significant, when comparing two surveys that both followed a vessel. This research highlights the ability of marine electrical resistivity to collect mobile data of suspended sediment, which is applicable to many geotechnical problems such as increased sediment load generated from deep draft vessels or sediment movement following extreme events. Furthermore, if used by engineers or port authorities, such data collection methods can support an asset management plan for the amounts of sediments to be redistributed.